Hi, my D7 (9G7101) is fitted with a MJK 4/2 180C-109 mag. My understanding (from D7 parts catalog) is that this mag was a replacement for the factory Eisemann mag and is (I believe) Cat part # 7F9923. I sent this mag to a local chap for test/rebuild, he rebuilt the mag & it now has a good spark. However, he has told me that the mag sparks when the rotor wiper is at the 10 o'clock & 2 o'clock positions which does not line up with the 2 carbon brushes in the cap. He believes that the wiper on the rotor would need to spark at the 9 o'clock & 3 o'clock positions for the cap. My understanding of mags wouldn't be great so I am just going on what the chap has told me, his opinion is that the distributor cap is the wrong one. I have attached some photos for reference.
Has anyone come across this issue before or        have any ideas?

Thanks a lot, Shane

is the cam set correctly to open the points? is it the correct points? have the points worn too much? is it the right rotor button?

Hi trainzkid88, thanks for the reply
I will ask the mag man to check your comments on the points, not sure how we would know if the rotor button was correct or not? I will ask him to check if there is a part number stamped on the back.
When I look at the MJK parts book (oldcroak.com/mjk-edition-c-d-parts-list/) there only appears to be 1 part # for the ROTOR, distributor RT52154PA and it is the same for both 360 deg and 180 deg versions 

The 109 is the correct replacement. You may have the wrong cap. 180 deg. uses DP52471, the 360 deg. uses a DP52442.

yes check all the part numbers are correct for the application. bosch,wico and lucas are the 3 most popular brands of mag in the world and there were versions within versions. robert bosch gmbh developed a lot of automotive technology and many companies licenced that tech. and it continues today.

Hi Perkinsengineer,
The Magneto cap you have pictured is true and correct for the MJK 4/2 180, the lower/centre brush contacts the centre of the rotor and the upper left and right brushes are in the 10 o'clock 2 o'clock arrangement to provide the odd firing of the earlier pilot motor firing pattern. The later even firing pattern on the MJK 4/2 360 magneto on the later engines is obtained by a 10 o'clock 4 o'clock arrangement. Those caps are still available. I hope this helps,
Darren

Thanks Neil. Its a tricky one, I don't know the history of the tractor or when it last may have ran (although I think it was a very long time ago!). 109 corresponds with 7F9923 which is listed in the parts book as correct for my serial number - which Old Magnet has confirmed. Although there may be several part #'s for the cap in the parts book, I believe there might be only 2 designs - 180deg (D6, D7, D8) and 360deg (D2, D4). I also have a D4 2T which has a good A-B mag, I believe this has a 360 deg cap (I believe the part # for this cap is DP52442) as the pony on the D2/D4 will fire at 360, 720 etc.. intervals so my next option is to see if this cap will fit the 109 mag. This makes no sense, as any images I have seen of the 109 or 311 (vertical flange version of the 109) has a cap like mine - the 360 deg cap has obvious offset HT lead spouts, but I am willing to try anything at this stage to get her going !

Here's a picture of the inside of a MJK 4/2 360 cap, and inside of a MJK 4/2 180 cap. 

 
Here's a picture of the inside of a MJK 4/2 360 cap, and inside of a MJK 4/2 180 cap.